1. Field of the Invention
The present invention relates to systems for positioning optical elements within a lithographic apparatus.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, light is directed to a patterning device, which can be referred to as a mask, a reticle, an array of individually programmable or controllable elements (maskless), or the like. The patterning device can be used to generate a circuit pattern corresponding to an individual layer of an IC, flat panel display, or other device. This pattern can be transferred onto all or part of the substrate (e.g., a glass plate, a wafer, etc.), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. The imaging can include the processing of light through the like. Other components or devices can exist in a lithographic apparatus, such as a projection system, which can include optical components such as mirrors, lenses, beam splitters, and that can also contain optical components, such as a multi-field relay (MFR), which contains optical components to divide a radiation beam into a number of individual beams prior to patterning.
Projection systems, such as those commonly incorporated into a lithographic apparatus, often include telescoping, variable zoom lens assemblies. In such assemblies, one or more densely-packed lens elements (and other optical elements) must be positioned accurately and reliably within a confined housing. In existing variable zoom lens assemblies, such positioning is generally performed using a slide mechanism coupled to one or more actuators. For example, a lens element within the variable zoom lens assembly can be supported within a housing and positioned along a slider using a single mechanical actuator.
However, the accuracy with which a lens element can be positioned in such assemblies is degraded by the friction, stiction, and low or variable stiffness that are characteristic of existing slide assemblies. Further, such friction, stiction, and low or variable stiffness affects even high-quality slides that employ low-friction materials, ball or roller bearings, or fluid bearings. Similar deficiencies are present in existing mechanical actuators that are coupled to these slide mechanisms, such as plain screws or roller or ball screws. Frictionless bearings, such as fluid bearings or magnetic levitation bearings, also tend to provide low stiffness when incorporated within existing slide assemblies.
In addition, any misalignment of the slide, or any defect in the straightness of the slide, produces a corresponding variation in the lateral position of the lens element or in its tip, tilt, or rotation. The use of a single actuator per optical element, which is common in existing positioning systems, does not allow for independent adjustment or correction of a lateral orientation of the lens element, or in its tip, tilt, or rotation.